multi: introduce interface for payment database

diff --git a/config_builder.go b/config_builder.go
index be56f96..af4a06e 100644
--- a/config_builder.go
+++ b/config_builder.go
@@ -925,9 +925,9 @@ type DatabaseInstances struct {
 	// InvoiceDB is the database that stores information about invoices.
 	InvoiceDB invoices.InvoiceDB
 
-	// KVPaymentsDB is the database that stores all payment related
+	// PaymentsDB is the database that stores all payment related
 	// information.
-	KVPaymentsDB *paymentsdb.KVPaymentsDB
+	PaymentsDB paymentsdb.DB
 
 	// MacaroonDB is the database that stores macaroon root keys.
 	MacaroonDB kvdb.Backend
@@ -1237,7 +1237,7 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 
 		return nil, nil, err
 	}
-	dbs.KVPaymentsDB = kvPaymentsDB
+	dbs.PaymentsDB = kvPaymentsDB
 
 	// Wrap the watchtower client DB and make sure we clean up.
 	if cfg.WtClient.Active {
diff --git a/payments/db/interface.go b/payments/db/interface.go
new file mode 100644
index 0000000..d3a7e76
--- /dev/null
+++ b/payments/db/interface.go
@@ -0,0 +1,91 @@
+package paymentsdb
+
+import (
+	"context"
+
+	"github.com/lightningnetwork/lnd/lntypes"
+)
+
+// DB represents the interface to the underlying payments database.
+type DB interface {
+	PaymentReader
+	PaymentWriter
+}
+
+// PaymentReader represents the interface to read operations from the payments
+// database.
+type PaymentReader interface {
+	// QueryPayments queries the payments database and should support
+	// pagination.
+	QueryPayments(ctx context.Context, query Query) (Response, error)
+
+	// FetchPayment fetches the payment corresponding to the given payment
+	// hash.
+	FetchPayment(paymentHash lntypes.Hash) (*MPPayment, error)
+
+	// FetchInFlightPayments returns all payments with status InFlight.
+	FetchInFlightPayments() ([]*MPPayment, error)
+}
+
+// PaymentWriter represents the interface to write operations to the payments
+// database.
+type PaymentWriter interface {
+	// DeletePayment deletes a payment from the DB given its payment hash.
+	DeletePayment(paymentHash lntypes.Hash, failedAttemptsOnly bool) error
+
+	// DeletePayments deletes all payments from the DB given the specified
+	// flags.
+	DeletePayments(failedOnly, failedAttemptsOnly bool) (int, error)
+
+	PaymentControl
+}
+
+// PaymentControl represents the interface to control the payment lifecycle and
+// its database operations. This interface represents the control flow of how
+// a payment should be handled in the database. They are not just writing
+// operations but they inherently represent the flow of a payment. The methods
+// are called in the following order:
+//
+// 1. InitPayment
+// 2. RegisterAttempt (a payment can have multiple attempts)
+// 3. SettleAttempt or FailAttempt (attempts can also fail as long as the
+// sending amount will be eventually settled).
+// 4. Payment succeeds or "Fail" is called.
+// 5. DeleteFailedAttempts is called which will delete all failed attempts
+// for a payment to clean up the database.
+type PaymentControl interface {
+	// InitPayment checks that no other payment with the same payment hash
+	// exists in the database before creating a new payment. However, it
+	// should allow the user making a subsequent payment if the payment is
+	// in a Failed state.
+	InitPayment(lntypes.Hash, *PaymentCreationInfo) error
+
+	// RegisterAttempt atomically records the provided HTLCAttemptInfo.
+	RegisterAttempt(lntypes.Hash, *HTLCAttemptInfo) (*MPPayment, error)
+
+	// SettleAttempt marks the given attempt settled with the preimage. If
+	// this is a multi shard payment, this might implicitly mean the
+	// full payment succeeded.
+	//
+	// After invoking this method, InitPayment should always return an
+	// error to prevent us from making duplicate payments to the same
+	// payment hash. The provided preimage is atomically saved to the DB
+	// for record keeping.
+	SettleAttempt(lntypes.Hash, uint64, *HTLCSettleInfo) (*MPPayment, error)
+
+	// FailAttempt marks the given payment attempt failed.
+	FailAttempt(lntypes.Hash, uint64, *HTLCFailInfo) (*MPPayment, error)
+
+	// Fail transitions a payment into the Failed state, and records
+	// the ultimate reason the payment failed. Note that this should only
+	// be called when all active attempts are already failed. After
+	// invoking this method, InitPayment should return nil on its next call
+	// for this payment hash, allowing the user to make a subsequent
+	// payment.
+	Fail(lntypes.Hash, FailureReason) (*MPPayment, error)
+
+	// DeleteFailedAttempts removes all failed HTLCs from the db. It should
+	// be called for a given payment whenever all inflight htlcs are
+	// completed, and the payment has reached a final terminal state.
+	DeleteFailedAttempts(lntypes.Hash) error
+}
diff --git a/routing/control_tower.go b/routing/control_tower.go
index c5af230..dd984c9 100644
--- a/routing/control_tower.go
+++ b/routing/control_tower.go
@@ -151,7 +151,7 @@ func (s *controlTowerSubscriberImpl) Updates() <-chan interface{} {
 // controlTower is persistent implementation of ControlTower to restrict
 // double payment sending.
 type controlTower struct {
-	db *paymentsdb.KVPaymentsDB
+	db paymentsdb.DB
 
 	// subscriberIndex is used to provide a unique id for each subscriber
 	// to all payments. This is used to easily remove the subscriber when
@@ -168,7 +168,7 @@ type controlTower struct {
 }
 
 // NewControlTower creates a new instance of the controlTower.
-func NewControlTower(db *paymentsdb.KVPaymentsDB) ControlTower {
+func NewControlTower(db paymentsdb.DB) ControlTower {
 	return &controlTower{
 		db: db,
 		subscribersAllPayments: make(
diff --git a/rpcserver.go b/rpcserver.go
index 8b4c1b1..7add215 100644
--- a/rpcserver.go
+++ b/rpcserver.go
@@ -7529,7 +7529,7 @@ func (r *rpcServer) ListPayments(ctx context.Context,
 		query.MaxPayments = math.MaxUint64
 	}
 
-	paymentsQuerySlice, err := r.server.kvPaymentsDB.QueryPayments(
+	paymentsQuerySlice, err := r.server.paymentsDB.QueryPayments(
 		ctx, query,
 	)
 	if err != nil {
@@ -7612,7 +7612,7 @@ func (r *rpcServer) DeletePayment(ctx context.Context,
 	rpcsLog.Infof("[DeletePayment] payment_identifier=%v, "+
 		"failed_htlcs_only=%v", hash, req.FailedHtlcsOnly)
 
-	err = r.server.kvPaymentsDB.DeletePayment(hash, req.FailedHtlcsOnly)
+	err = r.server.paymentsDB.DeletePayment(hash, req.FailedHtlcsOnly)
 	if err != nil {
 		return nil, err
 	}
@@ -7652,7 +7652,7 @@ func (r *rpcServer) DeleteAllPayments(ctx context.Context,
 		"failed_htlcs_only=%v", req.FailedPaymentsOnly,
 		req.FailedHtlcsOnly)
 
-	numDeletedPayments, err := r.server.kvPaymentsDB.DeletePayments(
+	numDeletedPayments, err := r.server.paymentsDB.DeletePayments(
 		req.FailedPaymentsOnly, req.FailedHtlcsOnly,
 	)
 	if err != nil {
diff --git a/server.go b/server.go
index b45c5b8..cfbc55d 100644
--- a/server.go
+++ b/server.go
@@ -336,11 +336,9 @@ type server struct {
 
 	invoicesDB invoices.InvoiceDB
 
-	// kvPaymentsDB is the DB that contains all functions for managing
+	// paymentsDB is the DB that contains all functions for managing
 	// payments.
-	//
-	// TODO(ziggie): Replace with interface.
-	kvPaymentsDB *paymentsdb.KVPaymentsDB
+	paymentsDB paymentsdb.DB
 
 	aliasMgr *aliasmgr.Manager
 
@@ -683,7 +681,7 @@ func newServer(ctx context.Context, cfg *Config, listenAddrs []net.Addr,
 		addrSource:     addrSource,
 		miscDB:         dbs.ChanStateDB,
 		invoicesDB:     dbs.InvoiceDB,
-		kvPaymentsDB:   dbs.KVPaymentsDB,
+		paymentsDB:     dbs.PaymentsDB,
 		cc:             cc,
 		sigPool:        lnwallet.NewSigPool(cfg.Workers.Sig, cc.Signer),
 		writePool:      writePool,
@@ -1007,7 +1005,7 @@ func newServer(ctx context.Context, cfg *Config, listenAddrs []net.Addr,
 		PathFindingConfig:   pathFindingConfig,
 	}
 
-	s.controlTower = routing.NewControlTower(dbs.KVPaymentsDB)
+	s.controlTower = routing.NewControlTower(dbs.PaymentsDB)
 
 	strictPruning := cfg.Bitcoin.Node == "neutrino" ||
 		cfg.Routing.StrictZombiePruning